Reflector for Acoustic Pressure Wave Head

ABSTRACT

The present invention relates to a reflector for an acoustic shock or pressure wave head, wherein the reflector comprises an acoustically reflective surface formed by a body of rotation, said body of rotation being formed by rotation of an elliptical segment about a rotation axis which extends through a focal point of the ellipse and encloses an angle α between 0.1° and 30° with the main axis of the ellipse.

FIELD OF THE INVENTION

The present invention relates to a reflector for an acoustic shock orpressure wave head and to an acoustic shock or pressure wave head with areflector.

BACKGROUND

Acoustic shock or pressure waves have been used, e.g., inelectrohydraulic lithotripsy to destroy hardened masses like kidneystones, bezoars or gallstones. An apparatus for generating pressurepulse/shockwaves is described, e.g., in U.S. Pat. No. 8,257,282. Theapparatus comprises a pressure pulse/shockwave source, a housingenclosing said pressure pulse/shockwave source, and an exit window fromwhich wave fronts of waves generated by said pressure pulse/shockwavesource emanate. The wave fronts have plane, nearly plane, convergent offtarget or divergent characteristics. An extracorporeal shockwave systemprovides a planar wave for the treatment of tissue. A parabolicreflector is provided in order to propagate the planar wave through amembrane and to the tissue of a human subject.

A reflector having the shape of an ellipsoid is known from, e.g., U.S.Pat. No. 4,702,249. Further reflectors are known from, e.g., DE 197 18511 A1, DE 253 89 60 C2 and DE 100 65 450 A1.

The techniques known in the prior art are all based on using eitherplane wave fronts or acoustic waves being focused into a small focalspot. However, while the use of a strongly focused wave may provideoptimum results in case of lithotripsy, this is not the case with othermedical applications such as, e.g., the application of acoustic waves toheart tissue during cardiac interventions. The application of shock waveto heart muscle needs to cover a certain area of the heart in order totreat the whole area affected by, e.g., an ischemia. Using focused shockwaves, each pulse will only cover a small area and a lot of shock wavepulses would be necessary to cover the whole area. This would extend thetreatment time and therefore the time the patient needs to stay undergeneral anesthesia increasing the patient's risk for side effects due toanesthesia. On the other hand the use of plane wave fronts would cover alarge area with each pulse but the energy flux density per pulse islimited due to the large area and the maximum energy output of thegenerator.

SUMMARY

It is an object of the present invention to provide an improvedreflector for acoustic shock or pressure waves which may, inter alia, beutilized in such advanced medical applications.

Accordingly, the present invention relates to a reflector for anacoustic shock or pressure wave head. The reflector comprises anacoustically reflective surface formed by a body of rotation. Said bodyof rotation is formed by rotation of an elliptical segment (of anellipse) about a rotation axis which extends through a focal point ofthe ellipse and encloses an angle α between 0.1° and 30° with the mainaxis of the ellipse.

The present invention is, inter alia, based on the idea to provide areflector whose focal region is expanded or widened as compared to afocal spot achieved by prior art reflectors. It has been realized thatthe healing process of a diseased tissue area is, at least in part,started in healthy tissue which is provided at the edge or surroundingof the diseased tissue area. It is thus advantageous to use a focusregion of maximum shock or pressure which provides sufficient acousticpressure to said edges or surrounding of the diseased area. Preferably,said focus region has the shape of a ring, a biconcave rotational solidor a discus. Such focal regions, which deviate from the focal spots ofprior art reflectors, may be achieved by adapting the various parametersof the body of rotation, which defines the acoustically reflectivesurface of the reflector.

Preferably, the short half-axis of the ellipse has a length between 10mm and 300 mm, more preferably between 15 mm and 200 mm and particularlypreferably between 20 mm and 100 mm.

Preferably, the ratio of the long half-axis to the short half-axis ofthe ellipse ranges between 1.05 and 2, more preferably between 1.1 and1.9, and particularly preferably between 1.2 and 1.8.

Preferably, the following relationship is fulfilled for the length L ofthe short half-axis measured in mm and the angle α measured in degrees:−0.003×L+0.8<α<−0.1×L+30.

Preferably, the angle α is at least 1°, more preferably at least 3°, andparticularly preferably at least 5°. Preferably, the angle is no largerthan 20°, more preferably no larger than 15°, and particularlypreferably no larger than 10°.

Preferably, the acoustically reflective surface comprises a materialwhose specific acoustic impedance is at least twice as large as thespecific acoustic impedance of water. One preferred material is metal,e.g. brass or stainless steel.

According to another aspect of the present invention, a reflector for anacoustic shock or pressure wave head is provided, wherein the reflectorcomprises an acoustically reflective surface formed by a body ofrotation, said body of rotation being formed by rotation of a parabolasegment about a rotation axis which extends through a focal point of theparabola and encloses an angle α between 0.1° and 30° with the axis ofthe parabola.

Preferably, the reflector comprises an aperture wherein the followingrelationship is fulfilled for the aperture's radius R measured in mm andthe angle α measured in degree: −0.003×R+0.8<α<−0.1×R+30.

Preferably, the angle α is at least 1°, more preferably at least 3°, andparticularly preferably at least 5°. Preferably, the angle is no largerthan 20°, more preferably no larger than 15°, and particularlypreferably no larger than 10°.

Preferably, the acoustically reflective surface comprises a materialwhose specific acoustic impedance is at least twice as large as thespecific acoustic impedance of water. One preferred material is metal,e.g. brass or stainless steel.

The present invention further relates to an acoustic shock or pressurewave head with a reflector according to any of the inventive aspectsdescribed above. The acoustic shock or pressure wave head comprises asource of acoustic shock or pressure waves, wherein said source isarranged in a focal point of the ellipse or the parabola.

Preferably, shock or pressure waves emitted by the source are reflectedon the acoustically reflective surface such that a focus region ofmaximum shock or pressure is formed outside the shock or pressure wavehead. Preferably, the focus region has the shape of a ring, a biconcaverotational solid or a discus. The optimum shape of the focus region maydepend on the specific medical application the waves are used for. Sincethe present invention allows for a wide spectrum of focus regions theuser of the inventive reflector or the medical practitioner may choosethe proper shape of the focus region for each specific application.

Preferably, the focus region is defined by the fact that the shock orpressure within said focus region decreases no more than 4 dB, morepreferably no more than 5 dB, particularly preferably no more than 6 dBwith respect to the maximum value.

Preferably, the diameter of the ring, the biconcave rotational solid orthe discus ranges between 5 mm and 30 mm, more preferably between 5 mmand 20 mm.

Preferably, the acoustic shock or pressure wave head further comprises amembrane which preferably has a specific acoustic impedancecorresponding to the specific acoustic impedance of water. Preferably,the volume enclosed by the acoustically reflective surface and themembrane is at least partially filled with a liquid, preferably water.Preferably, the source comprises two electrodes for spark discharge.Preferably, the liquid is enriched with conductive, semiconductive ornon-conductive particles.

The present invention further relates to a method of treating humantissue with an acoustic shock or pressure wave. The method comprisesproducing an acoustic shock or pressure wave by spark discharge betweenelectrodes which are supplied with electrical current, said electrodesbeing provided in a liquid medium such as water. The method furthercomprises focusing the produced acoustic shock or pressure waves bymeans of a reflector as described above such that the focus regioncorresponds with a treatment region of the human tissue. Preferably,said method of treatment is electrohydraulic lithotripsy. Preferably,the human tissue being treated comprises one or a combination of thefollowing tissues: heart tissue, muscle tissue, bone tissue, skin,tendons and ligaments, prostate tissue, kidney, pancreatic tissue, nervetissue.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are further elucidatedwith reference to the following Figures:

FIG. 1 shows a schematic cross-section of a reflector (including anelectrode setup) according to the prior art;

FIG. 2 shows a schematic cross-section of the reflector shown in FIG. 1indicating the propagation of the acoustic waves;

FIG. 3 shows a schematic cross-section of an acoustic reflector(including an electrode setup) according to a preferred embodiment ofthe present invention;

FIG. 4 shows a schematic cross-section of an acoustic reflector(including electrode setup) according to the prior art;

FIG. 5 shows a schematic cross-section of the acoustic reflector shownin FIG. 4 with the propagation of the acoustic waves being indicated;

FIG. 6 shows a schematic cross-section of an acoustic reflectoraccording to another preferred embodiment of the present invention;

FIG. 7 shows a series of sketches indicating the generation of the bodyof rotation underlying the invention; and

FIG. 8 shows schematic cross-sections through various focus regions.

DETAILED DESCRIPTION

FIG. 1 shows a schematic cross-section of a reflector for an acousticshock or pressure wave head according to the prior art. The reflectorcomprises an acoustically reflective surface or reflector wall 4 in theshape of an ellipsoid (or, to be precise, a part thereof) with a firstfocal point 21 and a second focal point 22. The distance between theaperture of the reflector and each of the two focal points 21, 22 isindicated with reference numeral 20. The aperture radius is indicatedwith reference numeral 1.

The reflector shown in FIG. 1 may be part of an acoustic shock orpressure wave head comprising a source of acoustic shock or pressurewaves. In case of FIG. 1, the source comprises two electrodes 6 and 7connected to a source of high voltage 8, 9 with, e.g., the positiveelectrode 6 being electrically insulated from the reflector wall 4 by ahigh voltage isolator 5 and, e.g., the negative electrode 7 being eitheron the same potential as the reflector 4 or electrically insulated fromit by another high voltage isolator (not shown). The source of acousticshock or pressure waves is arranged in the first focal point 21. Inother words, the tips of the electrodes 6 and 7 are provided on twoopposing sides of the first focal point 21 so as to generate a spark atthe very spot of the second focal point 21, if a high voltage is appliedto the electrodes 6 and 7.

The reflector volume 3, which is defined by the acoustically reflectivesurface or reflector wall 4 on the one hand, and a membrane 2 on theother hand is filled with a liquid such as water. The high voltage sparkover generated at the first focal point 21 ionizes the surrounding waterand thus generates a plasma which expands with supersonic speed. Thus,an acoustic shock or pressure wave is generated at the first focal point21. The emitted pressure waves or beams 30 (see FIG. 2) travel throughthe water provided within the reflector volume 3 and are reflected atthe acoustically reflective surface or reflector wall 4. The reflectedacoustic waves or beams 31 are all focused in the second focal point 22as shown in FIG. 2. The −6 dB focal region is indicated with referencenumeral 23 in FIG. 1. As can be seen, the focal region 23 of this priorart reflector has roughly the shape of a prolate spheroid with the polaraxis being substantially greater than the equatorial diameter.

FIG. 3 shows a schematic cross-section of a preferred embodiment of areflector for an acoustic shock or pressure wave head. The maincomponents of the reflector including the electrode setup of theacoustic shock or pressure wave head are identical to those of thereflector shown in FIG. 1. However, while the acoustically reflectivesurface 4 of the reflector shown in FIG. 1 is formed by a regularellipsoid (or a part thereof), the acoustically reflective surface 4 ofthe reflector shown in FIG. 3 is formed by a specific body of rotation.Said body of rotation is formed by rotation of an elliptical segment (ofan ellipse) about a rotation axis which extends through a focal point ofthe ellipse and encloses an angle α with the main axis of the ellipse.

The formation of this body of rotation is schematically shown in FIG. 7.View a) shows an ellipse with first and second focal points 21 and 22, ashort half-axis 43 and a long half-axis 44. The ellipsoidal reflectivesurface 4 shown in FIG. 1 would be achieved by rotation of the lowerhalf of the ellipse shown in view a) about the main axis 44 of saidellipse. However, according to the present invention, the axis ofrotation 42 encloses an angle α with the main axis 44 of the ellipse asshown in view b). As indicated in view c), a certain segment 40 of theellipse is chosen, which segment is to be rotated about the axis ofrotation 42. Said elliptical segment 40 need not correspond to exactly ¼of an ellipse. Rather, a first end 45 of the elliptical segment 40 ispreferably defined by the cross-section between the ellipse and the axisof rotation 42. The second end 46 of the elliptical segment 40 mayextend beyond the cross-section of the short half-axis 43 with theellipse as shown in view c). The perpendicular to the axis of rotation42 extending through the second end 46 will later on correspond to theaperture radius 1 of the reflector. The elements of view c) are againshown in view d) with the axis of rotation 42 being now aligned with thevertical. Rotating the elliptical segment 40 about the rotation axis 42,which still extends through first focal point 21 of the ellipseunderlying the elliptical segment 40 and which further encloses an angleα with the main axis 44 of the ellipse, will generate the acousticallyreflective surface 4 shown in FIG. 3.

Due to the fact that the axis of rotation 42 and the main axis 44 of theelliptical segment enclose an angle, the second focal point 22 due torotation about the axis of rotation 42 generates a ring-shaped focalregion.

This is further explained with reference to FIG. 3, which schematicallyshows a cross-section through the acoustically reflective surface 4formed by rotation of the elliptical segment 40 shown in view d) of FIG.7 about the axis of rotation 42. Looking at the right-hand side of FIG.3, it will be understood that each acoustic wave or beam 30 emitted fromthe first focal point 21 into the right-hand side of the cross-sectionalplane of the reflective surface 4 is reflected into the second focalpoint 21 a due to the fact that the first focal point 21 and the secondfocal point 22 a are focal points of the elliptical segment forming theright-hand side of the cross-sectional plane of the reflective surface4. Similarly, each acoustic beam 30 being emitted from the first focalpoint 21 into the left-hand side will be reflected onto second focalpoint 22 b. Since this is true for each cross-sectional plane throughthe reflective surface 4, a focal region in the shape of a ring is beingformed.

Mathematically, the focal region of a regular ellipsoid should be asingle point. Accordingly, the focal region of the reflector shown inFIG. 3 should correspond to a circular line. However, in real life, thepoint-like focus, due to imperfections, is expanded to the prolatespheroid shown in FIG. 1 (and, similarly, in view a) of FIG. 8) and thecircle-like focal region of FIG. 3 is expanded to a toroidal orring-like focal region as indicated in view e) of FIG. 8. Views a) to e)in FIG. 8 show the cross-sections of various focal zones or regionswhich may be generated by increasing the angle α enclosed by the axis ofrotation and the main axis of the ellipse underlying the ellipticalsegment from view a) (α=0°) to view e). If the angle α is larger than 0°and preferably at least 0.1° the focal region shown in view a) isexpanded or widened radially such that the prolate focal region shown inview a) becomes oblate as indicated in view b). Further increasing theangle α leads to a situation where the longitudinal extension of thefocal region is greater at the periphery of the focal region than in thecenter of the focal region as can be seen in view c). At the same time,the maximum intensity of the pressure profile is off axis, i.e.displaced radially from the axis of rotation. The larger the angle α thefurther is the focal region expanded radially (see view d)) until, e.g.,the −6 dB region forms a torus as shown in view e). Since the overallamount of energy or power transmitted does not change substantially,increasing the radial extension of the focal region at the same timereduces the longitudinal extension as schematically indicated in viewsa) to e) in FIG. 8.

Depending on the angle α and the particular profile of a single focalspot, the focal region can be manipulated to have the shape a discus asshown in view b) of FIG. 8, of a biconcave rotational solid as shown inviews c) and d) of FIG. 8, or even of a torus as shown in view e) ofFIG. 8.

As is evident from the above, the present invention allows for a veryprecise shaping of the focal region and, inter alia, provides thebenefit of allowing for a focal region with a local minimum in terms ofintensity and/or pressure at and/or close to the center of the focalregion. A focal region as that shown in views c) to e) of FIG. 8 allowsfor a particularly beneficial treatment of diseased tissue, becausetreatment is focused or maximized at the edges or surrounding of thediseased tissue thus stimulating the at least partly healthy tissuearound said diseased tissue. This specific shape adds a local pressuregradient between the two maxima of the focal zone occurring along across-section thereof. In views a) and b) of FIG. 8 the pressuregradient extends from the center of the focal zone radially to theperiphery. In views c) to e) of FIG. 8 an additional pressure gradientoccurs between the maximum pressure areas to the center of the focalzone and another one from the maximum to the periphery.

FIGS. 4 and 5 show a schematic cross-section of another reflector knownfrom the prior art, wherein the acoustically reflective surface 4 ofsaid reflector has a paraboloid shape. Apart from the fact that aparabola only has a single focal point 21, the reflector shown in FIGS.4 and 5 corresponds to that of FIG. 1. However, due to the parabolicshape of the acoustically reflective surface 4, the acoustic waves orbeams 30 emitted from the focal point 21 lead to a plane wave ofparallel reflected beams 31 as indicated in FIG. 5.

FIG. 6 shows a schematic cross-section of another preferred embodimentof a reflector according to the present invention. Said reflectorcomprises an acoustically reflective surface 4 formed by a body ofrotation. Similar to the body of rotation described above with respectto the elliptical segment, the body of rotation of the reflector shownin FIG. 6 is formed by rotation of a parabola segment about a rotationaxis which extends through the focal point of the parabola and enclosesan angle α with the axis of the parabola. The focus region achieved bythe reflector shown in FIG. 6 is, in particular, discus-shaped.

The reflector according to the present invention may be incorporatedinto any acoustic shock or pressure wave head known in the prior art.Even though the inventive reflector has been described with respect toreflection of acoustic shock or pressure waves, the invention mayanalogously be employed for reflection of other waves, in particularoptical waves.

1. A reflector for an acoustic shock or pressure wave head, wherein the reflector comprises an acoustically reflective surface formed by a body of rotation, said body of rotation being formed by rotation of a parabola segment of a parabola about a rotation axis which extends through a focal point of the parabola and encloses an angle α between 0.1° and 30° with a main axis of the parabola.
 2. The reflector according to claim 1, wherein the reflector comprises an aperture and wherein, dependent on the aperture's radius R measured in mm, the following applies for angle α measured in degrees: −0.003×R+0.8<α<−0.1×R+30.
 3. The reflector according to claim 2, wherein the angle α is no larger than 20°.
 4. The reflector according to claim 1, wherein the acoustically reflective surface comprises a material whose specific acoustic impedance is at least twice as large as the specific acoustic impedance of water.
 5. An acoustic shock or pressure wave head including the reflector according to claim 1, wherein the head further comprises a source of acoustic shock or pressure waves, wherein the source is arranged in the focal point of the parabola, wherein the acoustic shock or pressure waves emitted by the source are reflected on the acoustically reflective surface such that a focus region of maximum shock or pressure is formed outside the shock or pressure wave head.
 6. The acoustic shock or pressure wave head according to claim 5, wherein the focus region has the shape of a ring, a biconcave rotational solid or a discus.
 7. The acoustic shock or pressure wave head according to claim 5, wherein the focus region is defined by the fact that a shock or pressure within said focus region decreases no more than 6 dB with respect to the maximum shock or pressure.
 8. The acoustic shock or pressure wave head according to claim 5, further comprising a membrane, wherein a volume enclosed by the acoustically reflective surface and the membrane is at least partially filled with a liquid, and wherein the source comprises two electrodes for spark discharge.
 9. The acoustic shock or pressure wave head according to claim 8, wherein the liquid is water.
 10. The acoustic shock or pressure wave head according to claim 8, wherein the liquid is enriched with conductive, semiconductive or non-conductive particles. 